Touch screens or panels have become commonplace, being used extensively with tablet computers and smart phones, and these touch panels employ a wide variety of sensing technologies (e.g., resistive and capacitive touch panels). One such technology, namely IR sensing, can be seen in FIG. 1. In FIG. 1, the system 100 generally comprises a touch panel 102 that is typically secured to a housing (which has been omitted here for the sake of simplicity of illustration) that determines touch locations in the touch area 104 (which can, for example, include a liquid crystal display or LCD) by use of interference.
In operation, the controller 110 controls the driver 114 such that the transmitter 106 is able to generate IR beams over the touch area 104. As shown, the transmitter 106 (which generally comprises optical transmitter elements that can be IR light emitting diodes arranged along a portion of the periphery of the touch area 104 in this example. Located along a portion of the periphery and opposite the transmitter 106 is a receiver 108 that generally includes optical receiver elements (which can be photodiodes with each photodiode being aligned with at least one of the optical transmitter elements or IR light emitting diodes). These companion optical transmitter/optical receiver elements can be arranged to form a rectangular coordinate system such that each location within the touch area can be identified (with reasonable accuracy) to a horizontal and vertical position. As a result, the controller 110 can cause the driver 114 to “scan through” the optical transmitter elements. When an object (e.g., finger) that is opaque to IR is placed within the optical path of the companion optical transmitter/optical receiver elements, the beams (i.e., corresponding to the vertical and horizontal positions) are blocked. In synchronization with the driver 114, the controller 110 can cause the multiplexer 116 in AFE 120 select the appropriate optical receiver element so as to detect the blockage within the touch area 104. The photocurrents generated by the optical receiver elements in receiver 108 can then be converted to voltages with current-to-voltage (I2V) converter 118, and this information can be passed through the controller 110 (which can be a microcontroller) to the host 112 (which can, for example, be an applications processor within a mobile device).
Of interest here is the I2V converter 118, and an example configuration for the I2V converter 118 (which is labeled 118-A) can be seen in FIG. 2. In this example, amplifier 202 (which receives reference voltage REF1) along with resistor R1 and capacitor C1 function as a transimpedance amplifier, receiving a current pulse from one of the optical receiver elements through multiplexer 116. Amplifier 204 along with resistors R2 to R5 and capacitors C2 and C3 can function both as a filter (e.g., attenuate low frequency interference) and to compensate for leakage current (i.e., so as to not saturate amplifier 202 due to background light). Whenever multiplexer 116 is switched (as shown in FIG. 3), amplifier 204 should settle to a “new” leakage level, which may, for example, vary because the behavior of each optical receiver element varies or because of environmental conditions. This means that the loop bandwidths should be well above the biquad center frequency (e.g., about 1 MHz) to obtain rapid settling and that the gain bandwidth of amplifier 202 can be severely limited by the ratio of resistors R1 and R2. This ratio can also directly affect the settling speed (which is important as it relates directly to the scanning speed for touch detection) as it determines the feedback factor of the amplifier 202. Because resistor R2 is generally set by the bias photocurrent that should be sunk and because the resistor R1 is generally used to set receiver transresistance, this ratio tends to be large, indicating that amplifier 202 should be a fast amplifier. Additionally, the photocurrent can be several orders of magnitude larger than the received pulse, so, to receive a weak signal, amplifier 202 must have a high gain and settling of the photocurrent is significantly reduced due to saturation of amplifier 202.
In an alternative configuration (which is labeled 118-B in FIG. 4), the I2V converter 118-B employs cascaded front and back ends. In the front, amplifier 302 (which uses resistor R6) is coupled to multiplexer 116 and receives reference voltage REF2. From this configuration, it can be seen that there is no resistive loading at the input, meaning that amplifier 302 can be slower than, for example, amplifier 202 of FIG. 2. Feedback, in this example, can be accomplished through the use of automatic gain control (AGC) circuit 308 and current source 310 (which is usually a feedback transconductor implemented as a MOS transistor). The backend generally comprises amplifiers 304 and 306 (which each generally receive reference voltage REF3) that are cascaded with one another and employ resistors R7 and R8 and capacitors C4 to C7. A problem with this configuration is that the front end loop bandwidth varies depending on the photocurrent, meaning that it is difficult to stabilize. Moreover, there is a parasitic pole due to the transconductor control node that can negatively impact stability and settling speed.
Therefore, there is a need for an improved method and/or apparatus.
Some examples of conventional systems are: U.S. Pat. No. 8,031,094; U.S. Patent Pre-Grant Publ. No. 2009/0189878; U.S. Patent Pre-Grant Publ. No. 2011/0063154; U.S. Patent Pre-Grant Publ. No. 2012/0176343; U.S. Patent Pre-Grant Publ. No. 2012/0188205; U.S. Patent Pre-Grant Publ. No. 2012/0188206; and PCT Publ. No. WO2011066100.